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Unfortunately, cracked cylinder heads can be difficult to fix, and they are always expensive. the best thing would be to replace the cylinder head, but if the part isn't available, then that's a problem. It may be possible to fix the crack and keep the car drivable. I recommend calling some other shops and even parts stores to see if anyone can offer to a less expensive solution than replacing the entire motor. Sorry you're having such a mountain of trouble!
Going with the owner's manual is the best thing that you can do. The manufacturer has recommendations that are specific to your car, so you can ask the dealer to include what the manufacturer recommends and ask for them to justify everything above and beyond that.
That shudder associated with anti-lock brakes should only occur under circumstances when you are braking very hard and the wheels are starting to lock up. Wet conditions are a great example of when it happens more frequently. It is unlikely that the ABS system is coming into play when you are parking, which is probably at low speeds. Since you say that it happens only when the wheels are turned, it could be a problem with a steering component or a suspension component, and not be related to the brakes at all. If it happens every time, it should be easy for a good technician to diagnose and fix.
BMW 325i's are very reliable cars when they are well-cared for. The information I'm looking at says that BMWs made in the early 90s require what BMW calls and "Inspection I" at least every 10,000 Miles, according to factory specifications. This service includes changing the oil and oil filter, and possibly the air filter. What service you should perform now also depends on what was last performed by the prior owner. The best bet for keeping the car tip-top is to take it to a BMW expert and have them service your car based on what was most recently done and what it appears to need. The oil leak on the engine is definitely a concern. I wouldn't recommend mineral water--distilled water is best, or at least clean water with low mineral content in an emergency, but maintaining the right coolant mix is very important. BMWs are sensitive, high-performance cars, and investing in maintenance will save big bucks in the long haul.
The multimedia systems in these newest cars are hugely complex, so I don't know of any solution for you. Maybe try a Mercedes enthusiast site? There may be some seriously knowledgeable hackers out there. One thing to look forward to? I read recently that Mercedes is working on "split-screens" that will be factory equipment in there cars and show the driver the navigation screen and the passenger a DVD.
Unfortunately, transmission problems are really common these cars. On RepairPal's common problems page, it says "A delay in the transaxle engagement (greater than three seconds) may have multiple causes, including a defective pump, defective internal lip seals, valve body components, the Park Reverse Neutral Drive lever switch, and the output speed sensor." Having a shop check for these problems may save you from having to replace the entire transmission, but it's not a sure bet. Sorry!
That depends on the Taurus in question and what engine it has, as well as the wheels and even tires. I'm sure you can easily get it up over 100 miles and hour, though. Are you thinking of racing? The 2006 Taurus is rated 20 MPG by the EPA.
Auto Zone can be helpful, but they don't employ technicians who can diagnose and resolve problems. Your best bet is to take the car to a shop with a good diagnostic technician who can figure out what the error codes are and what they mean for your car.
This can be tricky, even for experienced technicians. The best bet is to buy a repair manual for your 528. You'll find detailed diagrams and instructions for how to find the fasteners and how to take off the panel. Another good resource are BMW enthusiast forums--you can find car-specific how to do it yourself information.
You can check what expert technicians say about the S420 on RepairPal at http://repairpal.com/mercedes-benz-s420-1997. Mercedes last for a long time, if well-cared for. Maintenance and repair are expensive--it is a Mercedes!--and one of the biggest problems to watch out for are oil leaks.